Jan 17, 2007
Superheated Water
Put simply, superheating involves raising the temperature of a liquid, for instance, beyond its boiling point without allowing it to vaporize. This can be done by heating water in a sealed container above 100 Celsius. There is an urban myth that has done the rounds for many years that it is possible to superheat the contents of a liquid-filled cup in a microwave and trigger a geyser of fluid when you remove it and stir. Who hasn’t received the spam-mail describing the 26-year old who was several disfigured by such an incident? Well, there are risks associated with all cooking, and heating a liquid in a microwave for long enough will produce a boiling temperature liquid and a container surface coated with scalding hot “condensation” that could cause you to jolt and end up splashing yourself with scalding liquid. Apparently, it happens, so be careful. However, I think it would be hard to actually “superheat” the liquid, although the guys in this video may have done just that using pure, distilled water or similar.
The liquid would have have no so-called “nucleation” points, specks of dust, particles, or whatever, even scratches inside the beaker to seed bubbles of steam. The water could very easily surpass its boiling point without actually boiling.
Like Snopes says, it is possible but takes a lot of effort to cause superheating in normal cup in the microwave. Nevertheless, it’s not worth risking a scalding by in an attempt to duplicate the above experiment with your morning coffee
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David Bradley said,
January 18, 2007 at 12:54 pm
Fellow blogger The Homely Scientist has issued a warning that my post is just plain wrong as he experienced a scalding after heating a drink too long in the microwave. So, yes, admittedly, it is possible to overheat (not necessarily superheat) a liquid and yes, it’s a good idea to be very careful with liquids and microwaves.
I’ve edited the post accordingly.
martin said,
January 19, 2007 at 9:37 am
In my personal experience a clean glass filled with ordinary (faucet) water boils differently in microwave than in a pot over a gas flame. Heated in a microwave owen the bubles in the water are much bigger and there’s a lot less of them compared to water boiling over a flame. I guess it’s because glass & water in it is much cleaner giving the water less opprtunities to create bubbles. As to why adding salt (or almost anything) to this “superheated” water may be dangerous: a) you create lots nucleaton centers and b) dissolving salts usually produces energy, rather than consuming it thus heaing the water ever more and sometimes, when the water is very close to its boiling point creating a dangerous small scale explosion, water discharging its excess energy in the now abundant nucleation centers.
Martin from Slovakia.